Photosynthesis 2.9, 8.3

Question 1

What is the general equation of photosynthesis?

Answer 1

6 CO2 + 6 H2O → C6H12O6 + 6 O2

Question 2

What are the ranges of wavelengths in each spectrum?

Answer 2

• visible light are between 400 and 700 nanometers
• longer wavelengths (infrared) = low energy
• shorter wavelengths (UV) = high energy
• plants absorb light because it is most abundant in nature

Question 3

Chlorophyll

Answer 3

• pigment
• absorbs red and blue light
• doesn’t absorb green → reflected
• used by plants to harvest light energy

Question 4

Thin layer chromatography

Answer 4

1. Blending plant tissue and gradually adding propanone
2. Putting pigment onto plastic strip
3. Marking the level of pigment on the tube
4. Adding running solvent.
5. After 5 minutes, taking plastic strip out and measuring the distance the solvent did and the pigments did. —> Rf is the fraction of these

From this value we can recognise different pigments

Question 5

Chloroplast

Answer 5

Outer membrane — separates from the rest (specialised compartment)

Chloroplast envelope — created by double membrane

Thylakoid — internal membrane (intense green)

Fluid filled spaces in thylakoid

Stroma — colourless fluid around thylakoid. Contains many enzymes

Grand — stacks of thylakoid

Starch grains and lipid droplets — if plant is photosynthesising quickly

Question 6

Photoactivation

Answer 6

• photosystems = chlorophyll and other pigments together to harvest light
  
  • Photosystem I and II
• centre → energy absorbed by chlorophyll is moved to special chlorophyll molecules
  
  • energy from light absorbed → e- excited
• chlorophyll molecule photoactivated
  
  • transfers e- to e- acceptor
• Photosystem II → e- acceptor = plastoquinone takes 2e-
  
  • plastoquinone moves to the other complex
    
    • hydrophobic → inside the membrane

2 photons of light → plastoquinone
1 chlorophyll → 2e-
for 2 plastoquinones: 4e- from 2 chlorophyll molecules

Question 7

Photolysis

Answer 7

• in thylakoid fluid
• photolysis = oxidation of water
  
  • for chlorophyll which lost 4e-

2 H2O —> O2 + 4 H+ + 4 e-

• oxygen is a waste product → diffuses away

Question 8

Where does light-dependent reaction occur?

Answer 8

• in thylakoid membrane (third membrane)

Question 9

The electron transport chain

In photosynthesis

Answer 9

• photophosphorylation = producing ATP from light
• in thylakoid
• Photosystem II → electron transport chain → Photosystem I
  
  • H+ move through ATP synthase
• plastoquinone carries electrons to the chain of electron carriers from Photosystem II

Question 10

Proton gradient

In photosynthesis

Answer 10

• electrons pass through electron transport chain
  
  • protons into thylakoid space → gradient created
• photolysis also adds to gradient

Question 11

Chemiosmosis (photosynthesis)

Answer 11

• H+ down the gradient through ATP synthase
• energy released → phosphorylates ADP
• electrons are finally accepted by plastocyanin in the thylakoid fluid

Question 12

Reduction of NADP

In photosynthesis

Answer 12

• Photosystem II passes electrons to plastocyanin
• re-excited in Photosystem I (photoactivation)
  
  • passed to ferredoxin → 2 e- to reduce NADP → NADPH
• 12 H+ pumped into thylakoid space
  
  • around 3 ATP molecules
  • 8 photons absorbed
• supply of NADP is out
  
  • electron moves back to chain of electron carriers
    
    • protons pumped and energy made = cyclic photophosphorylation

Question 13

Carbon fixation (step 1 of Calvin cycle)

Answer 13

• CO2 used to create carbohydrates
• ribulose bisphosphate (RuBP) 5 C molecule + CO2 → GP (glycerate-3-phosphate)
  
  • catalysed by rubisco = ribulose bisphosphate carboxylase
• a lot of rubisco in stroma to maximise carbon fixation

3 RuBP + 3 CO2 —> 6 GP

Question 14

Reduction of GP (glycerate-3-phosphate)

Answer 14

• CO2 + RuBP → GP (catalysed by rubisco)
  
  • ratio of H:O wrong
  • hydrogen added to GP (reduction)
    
    • 6 ATP —> 6 ADP + 6P (energy) and 6 NADPH —> 6 NADP+ + 6 H+ (reducing agent)
• ATP and NADPH from light-dependent reactions
  
  • 6 GP → 6 TP (or G3P) - triose phosphate

Question 15

Regeneration of RuBP

In Calvin cycle

Answer 15

• RuBP must be regenerated for new cycle
• 5 TP → 3 RuBP
  
  • only 1 TP left (after 3 cycles)
• energy 3 ATP → 3 ADP + 3 P

Question 16

Calvin’s lollipop experiment

Answer 16

• 12C was exchanged with 14C (radioactive)
• photosynthesis stopped at different time intervals t
  
  • radioactive carbon compounds found
• showed the steps of Calvin cycle

Question 17

Photosynthesis impact on the Earth

Answer 17

• oxygen as by-product
• Great Oxidation Event = rise in oxygen concentration in atmosphere
  
  • reduction in methane and CO2 concentration → greenhouse effect reduced
    
    • glaciation
• iron precipitated in water (oxidised)
  
  • rock formation
• multicellular organisms evolved = significant rise in O2 levels

Question 18

What are the limiting factors of photosynthesis?

Answer 18

Temperature, light intensity, carbon dioxide concentration
When below optimal level they slow down the reaction. Factor furthest from the optimum is the limiting one.